1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) and its manufacturing method.
2. Description of the Related Art
Conventionally, twisted nematic-type (TN-type) LCDs or electrically controlled birefringence-type (ECB-type) LCDs have been in wide use. However, because of the uniformly oriented alignment of liquid crystal molecules inside a pixel at the time of voltage application, the color tone changes depending on the viewing angle. In order to reduce this problem relating to the viewing angle, there are techniques to differentiate the oriented alignments of the liquid crystal molecules within each pixel or to divide the region of liquid crystal molecules into differently oriented regions. Using this oriented alignment division, the visual property of each region is compensated with those of the surrounding regions so that a display with a wide-angle field of view can be produced. In particular, when the vertical orientation method (one of the ECB-type methods) is employed, a faster response speed than the TN system can be obtained. Furthermore, due to the improved black display, attention has been focused on the vertically oriented alignment division techniques.
Techniques such as rubbing and optical orientation have been used to orient the liquid crystal molecules. However, because rubbing utilizes a treatment where the alignment layer comes into contact with a cloth in a wiping-like manner, problems can easily develop due to molecules breaking down from static electricity or the substrate being contaminated with foreign dust particles. On the other hand, because the optical orientation process utilizes a non-contact treatment to orient the alignment layer, the above problems related to rubbing techniques do not develop. Therefore, the optical orientation process is seen as a superior technique.
Techniques for performing oriented alignment divisions using the optical vertical orientation method are described in Japanese Patent Application Laid-open Nos. Hei 9-211468 and Hei 10-142608. In each of these techniques, a photo mask is used, and an optical orientation treatment is performed on each of the orienting regions based on their respective irradiation condition.
Here is a brief explanation of the techniques described in the above Japanese Patent Application Laid-open No. Hei 9-211468. First, the principle of this optical orientation technique will be explained using FIGS. 1(A) to 1(C). As shown in these figures, top of the substrate 1 is coated with an ultraviolet (UV) ray inducting alignment layer (in this case a vertical alignment layer made of a polyimide). On the surface of the alignment layer 18 are CH chains 19, and the average direction of these CH chains 19 is parallel to the normal of the substrate 1. However, the directions of the respective CH chains are dispersed (varied). In other words, it is considered that they face all directions within each imaginary plane parallel to the substrate surface. If UV rays irradiate from the direction 22 diagonal to the alignment layer 18, parts of the CH chains with their directions being parallel to the direction that the UV rays travel, absorb the UV rays, and thus are cut off or disintegrate. The degree of UV ray absorption depends on the direction of each CH chain on the alignment layer 18 as well as the direction of, the UV ray irradiance, with the highest level of absorption occurring when the direction 23 of the electric field of the UV ray coincides with the direction of the CH chain.
After the UV ray irradiation has occurred, a certain direction (marked with an xe2x80x98Xxe2x80x99 in FIG. 1(A)) of CH chain is cut off or disintegrated on the alignment layer 18, whereas the other (marked with an xe2x80x98Oxe2x80x99 in FIG. 1(A)) is left intact. Then, as shown in FIG. 1(B), the direction 21 parallel to the remaining CH chains leans away from the normal of the substrate 1.
As shown in FIG. 1(C), when the liquid crystal molecules 24 come into contact with the surface of the oriented film 18, they are slanted so as to run along in the same directions as CH chains 19. The same treatment as described above is applied to two substrates, and in order for the slant of the liquid crystal molecules on the surface of the alignment layer of each substrate to be somewhat parallel to each other, the two substrates are arranged to face each other maintaining a fixed interval between them. The liquid crystal filling between the substrates is liquid crystal with a negative dielectric anisotropy. Applying a certain voltage to the electrode forces the angle of the slant of the liquid crystal molecules 24 relative to the normal of the substrate to increase on their own from the initial angles, eventually becoming parallel to the substrate. In short, a uniform orientation is able to be obtained.
Next, the process of oriented alignment division will be explained. As stated above, the oriented direction is dependent upon the condition of the irradiating light: mainly the irradiating angle and the condition of the light polarization. Accordingly, in order to differentiate the oriented direction of each region, it is necessary to change the condition of the irradiating light in each region. It is common for a photo mask to be used to allow a specific light to hit a specified region, but not allow it to hit the other regions. The process of oriented alignment division can be executed by repeatedly and selectively irradiating light using the photo mask in proportion to the number of oriented alignment divisions.
Furthermore, in Japanese Patent Application Laid-open Hei 10-142608, other techniques besides the above techniques relating to alignment layer materials, the condition of light irradiance (light polarization, angle of light irradiance, volume of light irradiated), orientation of the liquid crystal, and mode of liquid crystal are mentioned, however, the process of oriented alignment division is the same as in the above technique.
Nevertheless, with the conventional technique described above, a problem arises where the alignment at the boundary between the divided regions easily becomes unstable. The reason for this is that regions of over-exposure and under-exposure tend to develop depending on how accurately the photo mask is matched. Also, because the liquid crystal cannot be regulated on these areas of over-exposure and under-exposure, it is easy for a poor orientation to occur. Moreover, those areas of poor orientation may be the core cause of the normally oriented regions of liquid crystal molecules becoming disturbed. This problem will be further discussed below using FIGS. 2(A) and 2(B) and FIGS. 3(A) and 3(B).
As described above, an optical orientation process is performed through the irradiation of UV rays diagonal to the alignment layer. Accordingly, a photo mask is used when an oriented alignment division is performed. In other words, as shown in FIG. 2(A), when subjecting the left region of the alignment layer 26, which is on top of the substrate 25, to an optical orientation process, the right region is masked by the photo mask 27 (the first condition). Under this first condition, UV rays 31 are irradiated from the diagonal direction 29. Next, as shown in FIG. 2(B), when optically orienting the right region of the alignment layer 26, which is placed on top of the substrate 25, the left region is masked by the photo mask 28 (the second condition). Under this second condition, UV rays 32 are irradiated from the diagonal direction 30. Through this process, the liquid crystal molecules in the left region on the surface of the alignment layer 26 are slanted to the left, whereas the liquid crystal molecules in the right region are slanted to the right. As a result, two orienting regions with different orientations are formed.
However, the following problems occur on the boundary of each region of the alignment layer that has gone through an oriented alignment division. FIG. 3(A) shows the case where the differing conditions of the UV rays 33 and 34 are both irradiated to the alignment layer resulting in developing a double-exposed region 35, whereas FIG. 3(B) shows the case where an unexposed region develops. It is considered that the former case is caused by the poor alignment accuracy of the photo mask or the diffraction resulting from diagonally irradiated light. The liquid crystal molecules above/on the double-exposed region 35 can possibly be slanted in different directions than just left or right according to the aforementioned optical orientation process. On the other hand, the liquid crystal molecules above/on the under-exposed region 36 are able to maintain their vertical orientation. Accordingly, each of the situations shown in FIGS. 3(A) and 3(B) causes the liquid crystal molecules to slant in different directions than expected when a certain voltage is applied. Moreover, the poorly oriented regions 35 and 36 adversely effect the areas where proper light irradiation has occurred so that the poorly oriented area might expand.
Besides the above problems with the aforementioned boundary, there is also a problem with the alignment accuracy with which the substrates are aligned and fixed. Namely, when a pair of substrates are each put through the optical orientation process, and then put face to face so that the oriented directions of one of the substrates can fit those of the other, a discrepancy in the position may occur due to the limitation of the alignment accuracy. In this case, the alignment layer on one of the substrates may not fit the oriented directions of the liquid crystal molecules on the surface of the alignment layer that is placed on the other substrate. As a result, in the same way as the previous case, a poorly oriented alignment may occur.
Japanese Patent No. 2778500 mentions a technique where the aperture is formed on the electrode at the boundary region of the oriented alignment division. However, yet this technique is one relating to the TN mode. It has nothing to do with a technique of the vertical orientation mode where liquid crystal molecules are either vertically or almost vertically pre-oriented and they turn to be oriented horizontally when a certain voltage is applied.
Besides this, according to Japanese Patent Application Laid-open Hei 6-0434461, the directions of vertically oriented liquid crystal molecules are controlled by a structure with the aperture on the electrode, but without performing an orientation treatment such as rubbing or an optical orientation. However, there is a problem with the response time of this technique. Namely, since with this technique, the orientation control is performed in only the region near the aperture of the electrode, the regions further away are later oriented through propagation. Moreover, since the liquid crystal molecules in the regions further away are vertically aligned at the beginning, they tend to move in random directions when the voltage is applied. Due to this, the orientation control using propagation is slowed down, and this increases the response time. The propagation delay becomes more obvious as the distant areas from the aperture become wider, or in other words, as the pixels increase in size.
Taking the above problems into account, the present invention is made. Accordingly, the objective of the present invention is to provide a newly structured LCD with the following characteristics: a superior display quality, a high-speed response time, and a low frequency of poorly oriented regions.
According to an aspect of the present invention, an LCD with each pixel being formed of a plurality of differently oriented regions of an alignment layer is provided, and is comprised of an electrode (3) with at least one aperture (5) formed along-the boundary between adjacent differently oriented regions in an alignment layer (10) that is deposited on top of the said electrode (3) and also in the said aperture (5), with the said adjacent, differently oriented regions orienting the respective liquid crystal molecules (14, 15, 16), wherein the shortest allowable width W of the said aperture (5) is equal to the width (X) of the defectively oriented region in the said boundary. An example of the said LCD is illustrated in FIG. 4.
According to an aspect of the present invention, an LCD with each pixel being formed of a plurality of differently oriented regions of an alignment layer is provided, and is comprised of an electrode (3) with at least one aperture (5) formed along the boundary between adjacent differently oriented regions in an alignment layer (10) that is deposited on top of the said electrode (3) and also in the said aperture (5), with the said adjacent, differently oriented regions orienting the respective liquid crystal molecules (14, 15, 16) to be vertical or almost vertical when no electric field is applied via the said electrode (3). An example of the said LCD is illustrated in FIG. 4.
According to an aspect of the present invention, a method of fabricating an LCD is provided, and is comprised of the following steps: an aperture forming step (S1) of forming at least one aperture (5) along a to-be-formed boundary on an electrode (3), which has been placed on top of a substrate (1), with the width W of the said aperture (5) being equal to or longer than the expected width (X) of the defectively oriented region in a boundary that is to be generated later; a depositing step (S2) of depositing an alignment layer (10) over the resultant surface processed in the aperture forming step; and a generation step (S2) of generating differently oriented regions, which orient respective liquid crystal molecules, and the said boundary, which is sandwiched between the said differently oriented regions, all in the said alignment layer (10). An example of the said method of fabricating an LCD is illustrated in FIG. 5.